Recently, with the development of portable electronic devices including mobile phones, notebook computers, and the like, along with the commercialization of electric vehicles and hybrid electric vehicles, the need for high capacity rechargeable batteries has rapidly increased. Particularly, since performance of such electric devices mainly depends on rechargeable batteries, there is high demand for high performance batteries.
A rechargeable battery is generally composed of a cathode, an anode, an electrolyte, and the like, and an anode active material is an essential component for supplying lithium ions in the battery. The anode active material serves to supply lithium cations to a cathode through electrochemical reaction and the development of anode active materials is known to be more difficult than that of cathode active materials. As a raw material for the anode active materials, lithium cobalt oxide (LiCoO2) is generally used in the art, but can affect cycle life of the lithium battery through phase change in a high voltage region during operation cycle of the battery (J. Electrochem. Soc., 139 (1992), 2091).
Although LiNi0.8Co0.15Al0.05O2, which has higher capacity than LiCoO2, is also receiving attention, LiNi0.8Co0.15Al0.05O2 active materials suffer from violent price fluctuation associated with the price instability of nickel. In particular, LiNi0.8Co0.15Al0.05O2 causes explosion of the battery due to thermal instability in a charged state (Electrochem. Solid State Lett. 7, A380-A383 (2004)).
As a result, manganese-based materials have received attention as an alternative material in view of stable supply of inexpensive raw materials, no toxicity, and electrochemical and thermal stability. In particular, lithium manganese oxides of a spinel structure, including LiMnO2, Li4Mn5O12, Li2Mn4O9, LiMn2O4, and Li1.1Mn1.9O4, have attracted. In particular, Li[(Ni0.5Mn0.5)1-xCox]O2 (0≦x≦0.5) having excellent thermal stability is a strong candidate for next generation high output and large capacity rechargeable lithium batteries.
Li[(Ni0.5Mn0.5)1-xCox]O2 exhibits relatively high capacity and excellent reversibility. However, since Li[(Ni0.5Mn0.5)1-xCox]O2 has a smaller amount of Co serving to increase electron conductivity of the material per se than LiCoO2, Li[(Ni0.5Mn0.5)1-xCox]O2 has unsatisfactory rate capability. In addition, since thermal instability of Li[(Ni0.5Mn0.5)1-xCox]O2 in a charged state is not overcome (Journal of the Electrochemical Society, 155, A374-A383 (2008)), applicability of Li[(Ni0.5Mn0.5)1-xCox]O2 to high output and large capacity battery systems is not sufficiently ascertained.
LiMn2O4 has been studied as an anode active material since it has a spinel structure and exhibits high operating voltage and relatively high reversible capacity. This material employs manganese, which is present in high concentration in the earth's crust, and thus is much cheaper than other active materials. Since this material has slightly lower reversible capacity than LiCoO2 and LiNiO2, there is a difficulty in using this material as an anode active material for a rechargeable lithium ion battery of a portable power source. However, LiMn2O4 has excellent thermal stability as compared with other anode active materials. For this reason, it is expected that LiMn2O4 will be applied to an anode active material for medium and large rechargeable lithium ion batteries due to stability thereof.
However, although LiMn2O4 or Li1.1Mn1.9O4 has good cycle life at room temperature, theses materials have a problem of a rapid decrease in capacity upon continuous charge/discharge operation at high temperature. In particular, dissolution of manganese increases at a high temperature of 40° C. or more, causing rapid deterioration in capacity (Electrochemical and Solid-State Letters, 8, A171 (2005)). Although various attempts, such as substitution of a fluorine atom into a oxygen site, have been made to solve the problem of capacity deterioration caused by dissolution of manganese at high temperature, the problem caused by the manganese dissolution has yet to be overcome (Journal of Power sources, 81-82, 458 (1999)). In other words, capacity deterioration caused by the manganese dissolution has not solved yet, despite substitution of manganese using various elements (Mg, Al, Co, Ni, Fe, Cr, Zn, Cu, etc.). (Journal of Power Sources, 68, 578 (1997); Journal of Power Sources, 68, 582 (1997); Solid State Ionics, 73, 233 (1994); Journal of Electrochemical Society, 143, 1607 (1996); Proceeding of 11th International Conference on Solid State Ionics, Honolulu, 1997, p. 23; Journal of Power Sources, 68, 604 (1997); Journal of Solid State Chemistries, 132, 372 (1997); Solid State Ionics, 118, 179 (1999); Chemistry of Materials, 7, 379 (1995); Journal of Electrochemical Society, 145, 1238 (1998); Materials Chemistry and Physics, 87, 162 (2004)); Journal of Power Sources, 102, 326 (2001))
Even in the case in which LiMn2O4 or Li1.1Mn1.9O4 is formed through surface coating or complex formation at a nanometer scale with stable MgO, Al2O3 and Co3O4, capacity deterioration caused by dissolution of manganese cannot be solved (Solid State Ionics, 167, 237 (2004); Electrochem. Solid-State Lett. 5 A167 (2002); Chem. Commun. 2001, 1074).
Therefore, there is a need for a new spinel type anode active material, which can suppress manganese dissolution in a spinel type LiMn2O4 or Li1.1Mn1.9O4 and has stable cycle lifespan at high temperature.